1. Field of the Invention
The invention relates to a method of optically discriminating between the phases of a moving three-phase fluid containing two first phases, e.g. liquid phases, of similar refractive indices, and a third phase, e.g., a gas phase, having a refractive index that is significantly different.
The invention also relates to apparatus for implementing the method.
The method and the apparatus of the invention can be used whenever it is desired to discriminate between the three phases of a moving fluid. A particular application lies in the oil industry, where it is common to find a single fluid coming from an underground deposit and comprising a mixture of liquid oil, of water, and of gas.
2. State of the Art
In the oil industry, it is essential to know as accurately as possible what fraction of the effluent from a well is constituted by liquid oil, and this applies both when testing and when producing from the deposit.
In practice, such knowledge is nevertheless very difficult to acquire accurately, particularly because the fluid extracted by a well generally contains three components (liquid oil mixed with water and with gas) which form three distinct phases above a certain depth.
Unfortunately, although various techniques exist that make it possible to distinguish between two phases contained in a fluid, the presence of three phases makes it necessary, at present, to use two detectors simultaneously. This gives rise to apparatus that is relatively complex and also provides a measurement of the oil fraction contained in the liquid which itself contains a relatively large amount of error. For example, one of the probes distinguishes the gas phase from the liquid phases while the other probe distinguishes between one of the liquid phases (generally water) and the other liquid phase and the gas phase taken together. The error inherent to the measurement is thus significantly increased when the fraction of oil present in the liquid is calculated on the basis of two measured fractions.
An illustration of the present state of the art is given by document EP-A-0 508 894 in which an optical probe and a radiofrequency probe are combined to form a single detector.
The presence of two different probes in a single detector also suffers from the drawback of inevitably situating the sensitive zones of the two probes in different locations. This is particularly true when the two probes are not integrated in a common detector, as is often the case. The sensitive zone of each of the probes can then be in the presence of a different phase of the fluid, such that interpreting results is particularly difficult. That constitutes another source of error which is additional to the preceding source of error in making measurements inaccurate.
In the particular case where phases are distinguished by means of an optical probe, the probe is generally constituted by the conical end of a silica optical fiber, as also shown in document EP-A-0 508 894. Given the refractive index of silica (about 1.46) and the refractive indices of gas (about 1) of water (about 1.33) and of liquid oil (about 1.5), the angle of the conical end is designed so that any incident light beam is reflected on the end surface of the fiber so long as said end surface is in gas, whereas on the contrary there is practically no reflection of the beam when the end surface of the fiber is in water or in liquid oil.
A conventional optical probe made in this manner thus provides good discrimination between gas and the liquid phases of the fluid, but it does not distinguish in any way between the liquid phases.